1. Field of the Invention
This invention relates to oil well instrumentation systems, and more particularly, to a system for measuring, recording and displaying the pressure and temperature at various depths in an oil well drill stem.
2. Description of the Prior Art
Oil well drilling is extremely expensive and thus it is desirable to choose drilling locations which have a relatively good possibility of providing sufficient yields to justify the drilling costs. In order to select optimum drilling locations it is necessary to know the properties of the subsurface geological structure. These properties are determined by measuring pressure at various depths in a test hole to generate pressure versus depth plots. The plots are then hydrodynamically analyzed to determine the continuity or discontinuity, both laterally and vertically, of pressure systems within the geologic column. Pressure is normally measured during a stabilized shut-in pressure buildup. In this technique, the pressure and temperature sensors are lowered through the drill stem and a packer is placed above the transducer to seal the drill stem. Pressure below the packer then rises to a stabilized level and the measurements are taken. Measurements are then repeated at a different depth with the packers moved to seal off different geologic zones. After samples are recovered from the drill stem at various depths, the hydrocarbon recoveries can be related to their respective hydrodynamic systems in order to approximate the location of the gas/water or oil/water contact.
Various types of geographical representations of pressure data can then be generated. A potentiometric surface map shows the potential of a given horizon to support a column of free-standing water of known density at a given point expressed in feet of water. A potentiometric surface map generally defines areas of continuous permability and indicates the presence of possible barriers between these areas which may constitute stratigraphic traps. A barrier to fluid migration is indicated by a rapid change in potentiometric values.
Another type of geographic representation of pressure data is the pressure deflection map. This is a map of pressure values at various points with respect to a key hydrodynamic system. Barriers to fluid migration may be inferred by sudden changes in the pressure deflection values.
The contour interval selected for either the potentiometric surface map or the pressure deflection map is limited largely by the measurement error inherent in the pressure measuring device. With conventional pressure measuring devices it is not possible to measure pressures with sufficient accuracy to allow relatively small contour intervals. This limitation may reduce the reliability and usefulness of such geographical representations of pressure data.
Drill stem pressure tests are also performed in order to measure the properties of an oil well and the surrounding structure in order to calculate the total production of the well as well as the optimum production rate. According to this technique, a packer is utilized to seal a drill stem and the subsequent pressure increase below the packer is measured. The rate of pressure increase provides an indication of the porosity of the structure the oil is in as well as the production rate. Also, flow from the well can be increased until pressure starts to drop thereby providing a good indication of the rate at which flow can be sustained.
In an interference test, pressurized water is injected into a first well and the pressure response in a different well is measured. In a pulse testing mode, the water is injected into a stimulus well at periodic intervals and the pressure is recorded in an observation well. Although pulse testing theory is well developed, the lack of an extremely sensitive pressure gauge has always limited practical applications because the effects of pressure at the observation well are usually small. The most important advantage of pulse testing is that transients observed as a result of the pulse stimulus are easily distinguished even in the presence of unrelated dynamic reservoir pressure behavior. The results of the pulse tests allow the calculation of in situ permability and formation thickness between wells.
The most common device currently used for oil well pressure tests are analog pressure transducers connected to conventional strip-chart recorders. These devices are not sufficiently accurate to be useful in many applications and it is difficult to accurately correlate the position of the markings on the strip-chart with time.
Another commonly used device is a self-contained pressure transducer and recorder which is lowered into the drill stem. The primary disadvantage of this device is that the pressure measurements cannot be read until the recording medium is processed at a distant location.
Although pressure is the most important measured property, temperature is also measured in order to normalize or calibrate the pressure measurements and to measure properties of fluids in the drill stem. For example, the pressure increase in the drill stem depends not only on the production rate of the well but also on the viscosity of the oil. In order to determine the true porosity of the structure surrounding the well and the oil well production rate, it is necessary to know the viscosity of the oil which can be inferred from knowing the temperature of the oil. Also, a knowledge of the characteristics of temperature variations can indicate the presence of a gas rather than a fluid.